X-rays reveal what makes the Milky Way move

A new survey by University of Hawaii astronomers has found that, in a tug-of-war of cosmic proportions, our Milky Way galaxy is being pulled toward the largest concentration of matter in the observable Universe. This finding is being presented today by UH graduate student Dale D. Kocevski and collaborators at the American Astronomical Society meeting held in Washington, D.C.

The UH scientists used a new X-ray survey to determine what region is winning the tug-of-war: a massive association of galaxies over 500 million light-years away. The study shows that our galaxy's journey through space is not entirely due to the pull of nearby galaxies, but is affected by much farther regions of the Universe than previously thought.

Kocevski worked with Dr. Harald Ebeling, Dr. R. Brent Tully, both also with the UH's Institute for Astronomy, and Dr. Chris R. Mullis, a UH alumnus who is now a research fellow at the University of Michigan.

Astronomers have long known that the Milky Way is moving toward the constellation Centaurus at a speed of 1.4 million mph, but the reason for the movement remained a topic of debate. Over 20 years ago, it was suggested that the motion was due to the gravitational pull of a nearby large concentration of matter dubbed the Great Attractor. The Great Attractor is what is known as a supercluster, that is, a group of clusters of galaxies, and was estimated to contain matter equal to more than over 10 million billion times the mass of the sun.

Until now, efforts to find the Great Attractor were hampered by its location in the "zone of avoidance," an area behind the plane of the Milky Way where gas and dust within our galaxy block much of the visible light from objects outside it. The new survey, Clusters in the Zone of Avoidance (CIZA), is the first to search for the X-ray signatures of galaxy clusters behind the Milky Way and investigate the nature of the Great Attractor. Due to the difficulty of observing through the Milky Way, this region was the final portion of the sky in which the cluster population had yet to be mapped.

"X-rays can penetrate even regions that are extremely obscured by gas and dust, and galaxy clusters are sources of X-rays. This is what prompted us to attempt to map the distribution of galaxy clusters behind the plane of the Milky Way using X-ray observations," explains Ebeling, who initiated the survey in 1998.

Kocevski and collaborators report finding far fewer massive cluster systems near the Great Attractor than would be expected given the region's proposed mass. "One of our goals was to uncover the true mass of the Great Attractor. What we found is that it is not that great after all," says Kocevski.Instead, the CIZA team identified a significant concentration of galaxies behind the Great Attractor, near the Shapley Supercluster, which lies 500 million light-years away or four times the distance to the Great Attractor region. The Shapley Supercluster, first identified in 1930 by Harlow Shapley, is the most massive association of galaxies out of the 220 identified superclusters in the observable Universe. It contains the equivalent of nearly 10,000 Milky Ways, or four times the amount of mass currently observed in the Great Attractor region.

With the galaxy cluster population mapped over the entire sky for the first time, Kocevski analyzed how all the clusters surrounding the Milky Way would affect it and found that only 44% of our galaxy's motion through space is due to the gravitational pull of galaxies in the nearby Great Attractor region. The remaining portion is the result of a large-scale flow in which much of the local Universe, including perhaps the Great Attractor itself, is being pulled toward the Shapley Supercluster.

The results confirm previous work, which suggested the Milky Way's motion was influenced by structures more distant than the Great Attractor, but this study is the first to reach this conclusion after having fully mapped the Great Attractor and regions behind it.

The finding resolves one of the long-standing problems associated with the Great Attractor. The presence of a massive overdensity relatively close to the Milky Way suggested that extreme mass concentrations such as the Great Attractor were fairly common in the Universe. This implied that the Universe contained much more matter than was measured by other means such as supernova Ia observations. The finding of a less massive Great Attractor and the large distance to the Shapley supercluster implies that extremely massive overdensities are rare in the Universe, which brings the suggested density of the Universe in line with the density established by independent means.

Preprints of papers submitted to The Astrophysical Journal on the Milky Way's motion and the CIZA survey can be found at:

This work was supported by the NASA Graduate Student Research Program.

Figure 1: This image of the core of the Shapley Supercluster shows a small portion of the thousands of galaxies that comprise Abell 3558, the galaxy cluster at the center of the largest mass concentration in the observable Universe. Image taken with NASA's Hubble Space Telescope; courtesy J. Blakeslee, Washington State University.

Figure 2: Two-dimensional projection of the cluster population within 800 million light-years of the Milky Way. Each blue halo represents a cluster of galaxies. Superclusters are located where multiple halos group together. The Milky Way's motion through space is due to a combination of the gravitational pull of the Great Attractor (small arrows) and the pull of the Shapley Supercluster, which produces a large-scale flow in which much of the Universe near our galaxy is streaming toward the more massive supercluster (large arrows).